Vehicle brakes are known whereby a brake rotor, such as a brake disc or a brake drum is attached to a vehicle wheel so as to rotate with that vehicle wheel. Non-rotating friction material, such as friction material on a brake pad or on a brake shoe can be frictionally engaged with the rotor so as to slow or stop the vehicle.
When it is not required to slow or stop the vehicle, the brake system is arranged so that a relatively small clearance, known as a running clearance, is provided between the rotor and the friction material. This running clearance is arranged to prevent brake drag. Brake drag is an unnecessary frictional drag, which causes increased fuel consumption, and increased wear of brake components. Accordingly brake drag is undesirable.
EP1000263 shows an example of a mechanically operated brake wherein the running clearance is automatically adjusted as the brake pads wear as a result of normal engagement and disengagement of the brake.
The automatic adjustment mechanism is designed to provide a predetermined running clearance for the brake. The running clearance is typically determined during the design process of the brake and depends upon various factors including rotor thickness, friction material thickness and other criteria of the brake assembly and associated vehicle.
One consideration when designing the running clearance is that the vehicle must be able to brake from its maximum speed or on a steep descent to a standstill and then, immediately be able to release the brakes and drive on. As will be appreciated, when a vehicle brakes from its maximum speed or on a steep descent to a standstill a considerable amount of kinetic energy is converted into heat energy by the brakes. This heat energy causes various brake components to heat up and therefore expand. In particular, where the brake is a disc brake, the brake disc and the friction material of the brake pads will expand. This expansion causes the running clearance of a hot brake to be smaller than the running clearance of a cool brake. Accordingly, the normal cool running clearance of a brake must be sufficiently large such that when the brake becomes hot (for example after being braked to a standstill from a maximum running speed of the associated vehicle) there still exists a running clearance to enable the vehicle to continue its journey. If the cool running clearance is too small, when the vehicle has braked to a standstill from its maximum speed, the friction material will be pressed into engagement with the rotor, and the vehicle will be unable to continue its journey, until such time as the brake components have cooled down.
However, when brakes which have a running clearance are applied, the friction material has to be moved over the distance of the running clearance before any braking force is applied to the rotor. Thus there is a short time delay between when the brake pads start to move, and when they start to apply a braking force.
EP0995923 shows an example of an electrically actuated adjuster mechanism which endeavours to maintain a constant running clearance.
Accordingly, in known brake designs the running clearance must be sufficient to ensure a running clearance when the brake is both hot and cool and to ensure that brake drag does not occur, but must not be so large so as to cause a significant time delay between when the brake pads start to move and when a brake force is applied to the rotor.
Thus, one object of the present invention is to provide an improved brake.